Twenty people were taken to hospitals Friday after two separate crashes involving San Francisco transit vehicles just minutes apart, fire officials said.

Crews were called to the city's Japantown neighborhood at 1:29 p.m. Friday when a bus collided with a dump truck, fire spokeswoman Jennifer Balestrieri said. Twenty people were assessed at the scene, 12 of whom were taken to hospitals. One of those injuries was considered serious, five moderate and six minor.

Balestrieri said crews were called to a light-rail crash in the Bayview neighborhood seven minutes later. Eight people were taken to hospitals with minor and moderate injuries after a light-rail train collided with a big rig.

Eleven people were assessed at the scene, but declined to be taken to a hospital.

Passenger Robert Williams told the San Francisco Chronicle that the train was full of passengers and had been slowing down as it pulled into a stop.

"The next thing you know, I heard a loud screech and the train came to a sudden stop," Williams said. "It was like an earthquake. Lots of people fell down."

It was not immediately clear what led to both crashes. Balestrieri said police will be heading up that investigation.

Calls to police and the transit agency were not immediately returned.

A San Francisco light-rail train crashed into another city light-rail train in July 2009, injuring 46 passengers and the operators of both vehicles. Federal investigators concluded that the driver of one of the trains had lost consciousness.